Aluminum alloy and aluminum alloy casting material

ABSTRACT

The present invention provides an aluminum alloy that has exceptional casting properties and that can exhibit high mechanical properties without being subjected to a heat treatment, and an aluminum alloy casting material. More specifically, the present invention provides: an aluminum alloy that has exceptional casting properties, and that has a high 0.2% proof stress and exceptional ductility without being subjected to a heat treatment; and an aluminum alloy casting material. The aluminum alloy according to the present invention is characterized by containing 7.0-9.0 mass % of Si, 2.0-4.0 mass % of Cu, 0.8-1.2 mass % of Mg, 0.3-0.5 mass % of Fe, 0.3-0.5 mass % of Mn, and. 2.0-4.0 mass % of Zn, the balance being Al and unavoidable impurities.

TECHNICAL FIELD

The present invention relates to an aluminum alloy for casting and analuminum alloy casting material made of the aluminum alloy.

PRIOR ARTS

Aluminum alloy materials are used for housings of portable electronicdevices and electronic terminals because they are lightweight and haveexcellent texture. The demand for thinness and weight reduction forthese portable electronic devices is increasing year by year, and higherstrength is required for the aluminum alloys used for the housings.

In particular, smartphones are often kept in a pocket or the like whennot in use, and bending stress is often applied in this situation. Thatis, aluminum alloys used for the housings of portable electronic devicesmust have high strength and ductility (toughness) in addition toexceptional casting properties.

On the other hand, for example, Patent Literature 1 (Japanese UnexaminedPatent Publication No. S48-32719) discloses, for the purpose ofobtaining an alloy having strength comparable to that of conventionalhigh-strength aluminum alloy for casting by taking advantage of theexceptional casting properties of Al-Cu-Si or Al-Si-Cu-Mg alloys, ahigh-strength aluminum alloy for casting having exceptional castingproperties comprising silicon of 7.5 to 1.2%, copper of 4.0 to 5.5%,magnesium of 0.2 to 1.0% by weight, the balance being aluminum andimpurities.

In the high-strength aluminum alloy for casting described in PatentLiterature 1, it is said that excellent mechanical properties can beimparted to the aluminum alloy casting material by subjecting agehardening treatment after performing solution treatment at about 500° C.

Further, Patent Literature 2 (Japanese Unexamined Patent Publication No.S60-57497) discloses, for the purpose of obtaining a heat-treatedhigh-strength aluminum alloy having good casting property, hightoughness, and excellent heat resistance, a heat-resistant high-strengthaluminum alloy comprising silicone of more than 6% to 13%, copper ofmore than 3% to 5.5%, zinc of more than 1% to 4%, magnesium of more than0.2% to 1% and antimony of more than 0.03% to 1% and the balance beingaluminum and impurities.

In the heat-resistant and high-strength aluminum alloy described inPatent Literature 2, it is said that when the Al-Si-Cu-Zn-Mg alloycontains more than 3% of copper, if antimony is added to the alloy,during aging treatment, the age hardening of the alloy is acceleratedand the strength of the alloy is significantly improved withoutsignificantly deteriorating the toughness, and the thermal shockresistance of the alloy is significantly improved.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No.S48-32719

Patent Literature 2: Japanese Unexamined Patent Publication No.S60-57497

SUMMARY OF THE INVENTION Technical Problem

In the high-strength aluminum alloy for casting described in PatentLiterature 1 and the heat-resistant high-strength aluminum alloydescribed in Patent Literature 2, though it is said that excellentmechanical properties are imparted in addition to exceptional castingproperties, in order to realize the mechanical properties, heattreatment such as artificial aging is essential.

However, the heat treatment process not only increases manufacturingcosts and manufacturing time, but also affects the dimensions and shapeof the aluminum alloy casting material. Particularly, since the housingsof portable electronic devices are thin and require high dimensionalaccuracy, it is desirable to achieve high strength and excellentductility without being subjected to heat treatment.

In view of the problems in the prior art as described above, an objectof the present invention is to provide an aluminum alloy and an aluminumalloy casting material that have exceptional casting properties and canexhibit high mechanical properties without being subjected to heattreatment. More specifically, the object of the present invention is toprovide an aluminum alloy and an aluminum alloy casting material thathave exceptional casting properties, high 0.2% proof stress andexcellent ductility without being subjected to heat treatment.

Solution to Problem

In order to achieve the above object, as a result of extensive study asto the composition range of aluminum alloys, the present inventors havefound it is effective that all of the addition amounts of Si, Cu, Mg,Fe, Mn and Zn are strictly controlled, and then reached the presentinvention.

Namely, the present invention provides an aluminum alloy which contains

-   -   Si: 7.0 to 9.0% by mass,    -   Cu: 2.0 to 4.0% by mass,    -   Mg: 0.8 to 1.2% by mass,    -   Fe: 0.3 to 0.5% by mass,    -   Mn: 0.3 to 0.5% by mass,    -   Zn: 2.0 to 4.0% by mass, and the balance being Al and        unavoidable impurities.

It is preferable that the aluminum alloy of the present inventioncontains at least one of Sr: 0.008 to 0.04% by mass, Be: 0.001 to 0.004%by mass, Ti: 0.05 to 0.005% by mass, and B: 0.01 to 0.005% by mass,

The present invention also provides an aluminum alloy casting materialwhich comprises the aluminum alloy of the present invention, and has a0.2% proof stress of 230 MPa or more and an elongation at break of 2.5%or more.

The aluminum alloy casting material of the present invention can developa 0.2% proof stress of 230 MPa or more and an elongation at break of2.5% or more after forming into a desired shape by casting without beingsubjected to heat treatment. A more preferable 0.2% proof stress is 240MPa or more, and a more preferable elongation at break is 3.0% or more.

Effect of the Invention

According to the present invention, it is possible to provide analuminum alloy and an aluminum alloy casting material that haveexceptional casting properties and can exhibit high mechanicalproperties without being subjected to heat treatment. More specifically,according to the present invention, it is possible to provide analuminum alloy and an aluminum alloy casting material that haveexceptional casting properties, high 0.2% proof stress and excellentductility without being subjected to heat treatment.

Embodiments for Achieving the Invention

Hereinafter, representative embodiments of the aluminum alloy andaluminum alloy casting material of the present invention will bedescribed in detail, but the present invention is not limited to theseexamples.

1. Aluminum Alloy

The aluminum alloy of the present invention is an aluminum alloy whichhas Si: 7.0 to 9.0% by mass, Cu: 2.0 to 4.0% by mass, Mg: 0.8 to 1.2% bymass, and Fe: 0.3 to 0.5% by mass, Mn: 0.3 to 0.5% by mass, Zn: 2.0 to4.0% by mass, and the balance being Al and unavoidable impurities. Eachcomponent will be described in detail hereinbelow.

(1) Essential Additive Element Si: 7.0 to 9.0% by Mass

Si has the effect of improving the casting property of aluminum and alsohas the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Si: 7.0% by mass or more.Conversely, when Si: 9.0% by mass or more, the crystallized eutectic Siand primary crystal Si tend to coarsen. When these compounds arecoarsened, they tend to serve as starting points for breakage, whichtends to lead to decrease in elongation. A more preferable additionamount of Si is 7.5 to 8.5% by mass.

Cu: 2.0 to 4.0% by Mass

Cu has the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Cu: 2.0% by mass or more.Conversely, when more than 4.0% by mass, the Cu-based crystallizedsubstances tend to coarsen, and the elongation tends to decrease.Further, when the content of Cu increases, the corrosion resistance alsodecreases. Furthermore, when anodized, the color tends to be yellowish.A more preferable addition amount of Cu is 2.5 to 3.7% by mass, andfurther preferable is 3.5% by mass or less.

Mg: 0.8 to 1.2% by Mass

Mg has the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Mg: 0.8% by mass or more.Conversely, when more than 1.2% by mass, the coarse compounds tend to beformed, and the elongation tends to decrease.

Though Si, Mg, and Cu are elements that precipitate as compounds duringaging treatment and contribute to precipitation strengthening, since thealuminum alloy of the present invention is mainly used as the non-heattreated materials, the strengthening mechanism of these elements isbasically solid-solution strengthening.

Fe: 0.2 to 0.5% by Mass

Fe has the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Fe: 0.2% by mass or more.It also has the effect of preventing seizure in mold casting such as diecasting. When more than 0.5% by mass, it is easy to form coarseneedle-like Al-Si, Fe, Mn)-based compounds that serve as fracturestarting points.

Mn: 0.3 to 0.5% by Mass

Mn has the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Mn: 0.3% by mass or more.It also has the effect of making the Al—(Si, Fe, Mn)-based compound intoparticles. Conversely, when more than 0.5% by mass, the Al—(Si, Fe,Mn)-based compounds tend to coarsen.

Zn: 2.0 to 4.0% by Mass

Zn has the effect of improving mechanical properties such as tensilestrength. This effect becomes remarkable when Zn: 2.0% by mass or more.Conversely, when more than 4.0% by mass, stress corrosion cracking tendsto occur. Further, discoloration and color unevenness are likely tooccur when subjecting to anodized film treatment.

(2) Optional Additive Element Sr: 0.008 to 0.04% by Mass

Sr has the effect of making the eutectic Si fine and granular, and thiseffect is remarkable when Sr: 0.008% by mass or more. When added morethan 0.04% by mass, since the effect is not significantly improved, itis preferably less than 0.04% by mass.

Be: 0.001 to 0.004% by Mass

Be has the effect that an oxide film is formed on the surface of themolten metal when melted, and the depletion of other elements such as Mgcan be suppressed. Further, it also has the effect of suppressing theblackening of the casting surface. This effect becomes remarkable whenBe is 0.001% by mass or more. When added more than 0.004% by mass, sincethe effect is not significantly improved, it is preferably less than0.004% by mass.

Ti: 0.05 to 0.005% by Mass

Ti mainly contributes to toughness by making the structure fine. Whenless than the lower limit, the effect is small, and when containingbeyond the upper limit, since it is already sufficiently fine and has noeffect, and in addition thereto, when adding excess amount, since coarsecrystalline compounds are formed to give adverse effect on elongation,it is necessary to limit it within the above range.

B: 0.01-0.005% by Mass

B mainly contributes to toughness by making the structure fine. Whenless than the lower limit, the effect is small, and when containingbeyond the upper limit, since it is already sufficiently fine and has noeffect, and in addition thereto, when adding excess amount, since coarsecrystalline compounds are formed to give adverse effect on. elongation,it is necessary to limit it within the above range.

As long as the effects of the present invention are not impaired, themethod for producing the aluminum alloy of the present invention is notparticularly limited, and conventionally known various productionmethods may be used.

2. Aluminum Alloy Casting Material

The aluminum alloy casting material of the present invention is made ofthe aluminum alloy of the present invention, and is characterized byhaving a 0.2% proof stress of 230 MPa or more and an elongation at breakof 2.5% or more. A more preferable 0.2% proof stress is 240 MPa or more,and a more preferable elongation at break is 3.0% or more.

The excellent mechanical properties are basically realized by rigorouslyoptimizing the composition, and the mechanical properties are ownedwithout depending on the shape and size of the aluminum alloy castingmaterial and further without depending on the portion and direction ofthe aluminum alloy casting material.

Further, the aluminum alloy casting material of the present inventioncan develop a 0.2% proof stress of 230 MPa or more and an elongation atbreak of 2.5% or more without being subjected to heat treatment such asaging treatment.

As long as the effects of the present invention are not impaired, theshape and size of the aluminum alloy casting material are notparticularly limited, and can he used as various conventionally knownmembers. Examples of such members include an electronic terminalhousing.

Further, as long as time effects of the present invention are notimpaired, the method for producing the aluminum alloy casting materialof the present invention is not particularly limited, and the aluminumalloy of the present invention may be cast by various conventionallyknown methods. Furthermore, the casting material by using the alloy ofthe present invention has excellent mechanical properties, particularlytoughness, even without heat treatment, but heat treatment such as agingtreatment may be performed. When aging treatment is performed, highermechanical properties can be obtained due to precipitation strengtheningof compounds such as Si, Mg, Cu, and Zn.

The representative embodiments of the present invention have beendescribed above, but the present invention is not limited to these, andvarious design changes are possible, and all such design changes areincluded in the technical scope of the present invention.

EXAMPLE Example

In Table 1, aluminum alloys having the compositions described asExamples 1 to 5 were produced by melting and die-cast at a castingpressure of 120 MPa, a molten metal temperature of 730° C., and a moldtemperature of 170° C. The mold shape is a plate of 55 mm×110 mm×3 mm.The aluminum alloy has excellent die-casting property, and a goodaluminum alloy casting material (die-cast material) was obtained. Theunit of the numerical values shown in Table 1. is % concentration bymass.

TABLE 1 Si Cu Mg Fe Mn Zn Be Sr Al Ex. 1 8.0 3.0 1.0 0.4 0.4 3.0 0.0030.020 Bal. Ex. 2 9.0 2.2 1.2 0.3 0.5 2.1 0.001 0.025 Bal. Ex. 3 7.2 3.90.8 0.5 0.3 3.9 — 0.038 Bal. Ex. 4 8.2 3.1 0.9 0.4 0.4 3.1 — 0.008 Bal.Ex. 5 7.7 2.8 1.1 0.5 0.5 2.8 0.004 0.012 Bal. Com. Ex. 1 11.0 4.5 0.80.25 1.0 — — 0.015 Bal. Com. Ex. 2 11.0 4.5 1.0 0.4 1.0 — — 0.010 Bal.Com. Ex. 3 11.0 4.5 1.0 0.4 1.0 1.0 — 0.010 Bal. Com. Ex. 4 10.5 4.5 1.00.4 1.0 3.0 — 0.010 Bal. Com. Ex. 5 10.0 4.5 1.0 0.4 1.0 5.0 — 0.010Bal. Com. Ex. 6 9.0 3.5 2.0 0.8 — — 0.003 0.020 Bal. Com. Ex. 7 9.0 3.52.0 0.8 — 3.0 0.003 0.020 Bal. Com. Ex. 8 9.0 3.5 2.0 0.8 — 5.0 0.0030.020 Bal. Com. Ex. 9 9.0 3.5 1.0 0.8 — — — 0.020 Bal. Com. Ex. 10 9.03.5 1.5 0.8 — — — — Bal. Com. Ex. 11 8.0 3.0 0.5 0.4 0.4 — 0.003 0.020Bal. Com. Ex. 12 8.0 3.0 0.5 0.4 0.4 3.0 0.003 0.020 Bal. Com. Ex. 138.9 3.6 0.51 0.78 <0.01 <0.01 0.003 0.020 Bal. Com. Ex. 14 8.9 3.6 1.10.79 <0.01 <0.01 0.003 0.020 Bal. Com. Ex. 15 8.8 3.6 1.5 0.79 <0.01<0.01 0.003 0.020 Bal. Com. Ex. 16 8.8 3.7 0.79 0.81 <0.01 <0.01 0.0030.020 Bal. Com. Ex. 17 7.3 3.6 0.6 0.76 <0.01 <0.01 0.003 0.020 Bal.Com. Ex. 18 7.3 3.6 0.9 0.77 <0.01 <0.01 0.002 0.020 Bal. Com. Ex. 197.3 3.6 1.2 0.77 <0.01 <0.01 0.003 0.020 Bal. Com. Ex. 20 1.9 <0.01 7.10.19 0.7 <0.01 0.003 — Bal. Com. Ex. 21 6.8 3.0 1.0 0.4 0.6 1.8 0.0030.020 Bal. Com. Ex. 22 7.9 1.7 0.9 0.4 0.1 2.9 0.003 0.020 Bal.

A No. 14B test piece defined in JIS-Z2241 was taken from each of theobtained aluminum alloy casting materials, and when the tensile test wasperformed at room temperature, the values shown in Table 2 of thetensile strength, the 0.2% proof stress and elongation at break wereobtained. Further, when the Rockwell hardness of the obtained aluminumalloy casting materials were measured, the values shown in Table 2 wereobtained. Here, the aluminum alloy casting materials were die-cast asthey were, and was not subjected to heat treatment such as agingtreatment.

TABLE 2 Tensile 0.2% Proof strength stress Elongation Hardness (MPa)(MPa) (%) (HRB) Ex. 1 369 241 3.3 73 Ex. 2 362 237 2.7 — Ex. 3 383 2472.6 — Ex. 4 365 242 3.2 — Ex. 5 371 245 3.0 — Com. Ex. 1 336 230 1.6 77Com. Ex. 2 362 237 2.0 77 Com. Ex. 3 317 231 1.2 76 Com. Ex. 4 347 2411.6 78 Com. Ex. 5 330 251 1.3 79 Com. Ex. 6 357 298 1.1 77 Com. Ex. 7385 301 1,5 78 Com. Ex. 8 366 302 1.0 79 Com. Ex. 9 357 239 2.2 76 Com.Ex. 10 385 253 1.6 77 Com. Ex. 11 — — — 59 Com. Ex. 12 — — — 59 Com. Ex.13 328 227 2.4 — Com. Ex. 14 328 271 1.5 — Com. Ex. 15 333 325 1.0 —Com. Ex. 16 333 241 1.8 — Com. Ex. 17 316 207 2.4 — Com. Ex. 18 347 2442.4 — Com. Ex. 19 350 282 1.5 — Com. Ex. 20 294 190 4.1 — Com. Ex. 21366 241 2.2 — Com. Ex. 22 349 225 2.3 —

Comparative Example

Comparative aluminum alloy casting materials (die cast materials) wereobtained in the same manner as in the Examples, except that the moltenmaterial was prepared so as to have the components described in Table 1as Comparative Examples 1 to 22. Further, the tensile properties andRockwell hardness were measured in the same manner as in Examples. Thevalues obtained are shown in Table 2. In addition, when there is nodescription of a numerical value, it means that the measurement was notperformed.

Comparing the tensile properties of the aluminum alloy casting materialsobtained in Examples and Comparative Examples, it can be seen that onlythe aluminum alloy casting materials obtained in Examples have a 0.2%proof stress of 230 MPa or more and an elongation at break of 2.5% ormore. Further, it can be seen that Example 1 with Sr added has highertensile strength and elongation than Example 4 with no Sr added(extremely low Sr content).

The aluminum alloy casting materials having the compositions ofComparative Examples 1 to 5, which contain a large amount of Si, Cu andMn, exhibit a high 0.2% proof stress, but the elongation at break is2.0% or less. Further, the elongation at break of the aluminum alloycasting materials having the compositions of Comparative Examples 6 to10 and Comparative Examples 13 to 19, which contain a large amount ofFe, does not reach 2.5%.

Further, the hardness of the aluminum alloy casting materials having thecompositions of Comparative Example 11, in which the amount of Mg addedis small and does not contain Zn, and Comparative Example 12, in whichthe amount of Mg added is small, are low values, and it can be seen thatsufficient strength is not obtained.

Furthermore, the aluminum alloy casting material having the compositionof Comparative Example 20 with low Si and Cu contents has an elongationat break of 2.5% or more, but a low 0.2% proof stress. Further, inComparative Example 21, in which the Si and Zn contents are low and theCu and Mn contents are high, the tensile strength and 0.2% proof stressare high, but the elongation at break is as low as less than 2.5%.Further, in Comparative Example 22, in which the Cu and Mn contents arehigh, the elongation at break is as low as less than 2.5%, and inaddition the 0.2% proof stress does not reach 230 MPa.

From the above results, in order to develop the 0.2% proof stress of 230MPa or more and the elongation at break of 2.5% or more withoutsubjecting the aluminum alloy casting material to heat treatment, it canbe seen that it is necessary to strictly control the addition amounts ofSi, Cu, Mg, Fe, Mn and Zn.

1. An aluminum alloy comprising: Si: 7.0 to 9.0% by mass, Cu: 2.0 to4.0% by mass, Mg: 0.8 to 1.2% by mass, Fe: 0.3 to 0.5% by mass, Mn: 0.3to 0.5% by mass, Zn: 2.0 to 4.0% by mass, and the balance being Al andunavoidable impurities.
 2. The aluminum alloy according to claim 1,which contains at least one of Sr: 0.008 to 0.04% by mass, Be: 0.001 to0.004% by mass, Ti: 0.05 to 0.005% by mass, B: 0.01 to 0.005% by mass.3. An aluminum alloy casting material which comprises the aluminum alloyaccording to claim 1, has a 0.2% proof stress of 230 MPa or more and anelongation at break of 2.5% or more.